1. Field of the Invention
The invention relates to a mirror arrangement for guiding a laser beam in a laser system in accordance with the preamble of claim 1, to a laser system having such a laser arrangement, and to a beam-guiding method for a laser beam in accordance with the preamble of claim 15.
2. Description of the Background Art
In laser systems for generating ultrashort pulses, that is to say pulses with a pulse duration in the femtosecond or picosecond range, there is often a need for large distances between the end mirrors, for example when low pulse repetition rates are necessary, that is to say typically less than ˜30 MHz. Aside from the large length (>4 m), resonators of such laser systems are to have a compact design, high insensitivity to unintended adjustment of the optical elements, for example owing to temperature changes, mechanical vibrations etc., as well as the possibility of simple implementation.
An example of such laser systems is mode-coupled laser arrangements employing the principle of pulse decoupling or cavity dumping, for example with Nd:YVO4 as laser material, and average powers of between 7.8 W and pulse energies of 15.6 μJ given repetition rates of 500 kHz and 10 W as well as 10 μJ given 1 MHz. Such laser systems are used, for example, for material processing.
Various methods are described in the prior art in the case of which two or more mirrors are arranged so that an optical beam or laser beam is reflected multiply between these mirrors, and it is thereby possible to increase the total path length on a small base surface.
These arrangements are usually named after the authors of the first reports, for example White cell (J. U. White, J. Opt. Soc. Am. 32, 285 (1942)), Hanst cell (P. L. Hanst, Adv. Environ. Sci. Technol. 2, 91 (1971)) or the likely most used cell, the Herriott cell (D. R. Herriott and H. J. Schulte, Appl. Opt. 4, 883 (1965) and U.S. Pat. No. 3,437,954). By way of example, the Herriott cell is described in more detail below.
Such a Herriott cell with multiple passes, or in a multipass arrangement, consists in the simplest arrangement of two mirrors as end mirrors defining the cell, at least one of them having a concave surface, which are arranged at a specified distance from one another. The mirror arrangement itself forms an optical resonator in which a beam is multiply reflected and repeats itself after a specific number of passes. There is formed in this case at the end mirrors a circulating pattern of reflection points lying on an ellipse or a circle. The beam is typically decoupled again from the mirror arrangement after a single pass of the cell, that is to say before the imminent identical repetition of the beam path. For the purpose of decoupling in the case of use in optical resonators, this closed beam path in the cell can be broken up either by separate mirrors or by holes at one of the end mirrors (see, for example, Kowalevicz et al., “Design principles of q-preserving multipass-cavity femtosecond lasers” J. Opt. Soc. Am. B, vol. 23, no. 4, April 2006).
Again, the publications of Kowalevicz et al. (“Generation of 150 nj pulses from a ultiple-pass cavity Kerr-lens mode-locked Ti:AL201 oscilator” Optic Letters Opt. Soc. Am., vol. 23, no. 17, September 2003) and Sennaroglu A, Fujimoto J. G. et al. (“Compact Femtosecond Lasers Based on Novel Multipass Cavities” IEEE Journal of Quantum Electronics, vol. 40, no. 5, May 2004) show examples for known Herriott cells with multiple passes, in the case of which, after a “circulation” (performed either clockwise or counterclockwise) of the reflection points on the end mirrors, a retroreflective element permits the beam to run back into itself.
Another beam-folding mirror arrangement is, for example, known from EP 1 588 461, in the case of which two reflecting planar surfaces are arranged so that the laser beam is multiply reflected at each of the reflecting surfaces, and the beam path has a beam entering the folding device and a beam exiting the folding device, the reflecting surfaces being oriented to one another with an aperture angle of greater than 0°. Such a linear arrangements leads therefore to a zig-zag profile of the beam path, the reflection points lying in a lines on the two reflecting surfaces, and having a varying spacing. In this arrangement, the space available for beam folding is therefore used only in a plane, that is to say in two dimensions.
In previous mirror arrangements, the number of the reflections is therefore restricted to one pass, or beam folding is performed only in a plane.
One object of the present invention consists in providing an improved laser system, in particular a diode-pumped, mode-coupled laser system.
A further object consists in providing such a laser system that has a greater compactness and/or greater robustness.
These objects are achieved, and/or the solutions are developed, by the subject matters of claims 1 and 15, or of the dependent claims.